pgsql: doc: add pg_prewarm example

doc: add pg_prewarm example

Discussion: /messages/by-id/20220618085541.ezxdaljlpo6x7msc@home-desktop

Author: Dong Wook Lee

Backpatch-through: 11

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f02f0b8cea5dddfa8ff5e0e26d2ece2052c4f0fe

Modified Files
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doc/src/sgml/pgprewarm.sgml | 13 +++++++++++++
1 file changed, 13 insertions(+)